service: Fix the issue that cannot update only TTL of the min service safepoint (#2633)

[ti-srebot]

cherry-pick #2633 to release-4.0

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What problem does this PR solve?

Fixes #2628

Do not skip updating service safepoint when the requested safepoint equals to the minimum safepoint currently.

What is changed and how it works?

Check List

Tests

• Unit test

Related changes

• Need to cherry-pick to the release branch
  • release-4.0

Release note

• Fixes the issue that sometimes service safepoint can't be set properly, which may make BR and dumpling fail.

[ti-srebot]

@MyonKeminta please accept the invitation then you can push to the cherry-pick pull requests.
https://github.com/ti-srebot/pd/invitations

[lhy1024]

/merge

[ti-srebot]

/run-all-tests

[ti-srebot]

@ti-srebot merge failed.

[lhy1024]

/merge

[ti-srebot]

Your auto merge job has been accepted, waiting for:

• 2627

[ti-srebot]

/run-all-tests

[lhy1024]

/merge

[ti-srebot]

/run-all-tests

[codecov-commenter]

Codecov Report

Merging #2635 into release-4.0 will decrease coverage by 0.05%.
The diff coverage is 100.00%.

@@               Coverage Diff               @@
##           release-4.0    #2635      +/-   ##
===============================================
- Coverage        77.11%   77.05%   -0.06%     
===============================================
  Files              205      205              
  Lines            22115    22115              
===============================================
- Hits             17053    17041      -12     
- Misses            3761     3769       +8     
- Partials          1301     1305       +4     

Impacted Files	Coverage Δ
server/grpc_service.go	58.49% <100.00%> (-0.78%)	⬇️
server/schedulers/shuffle_hot_region.go	63.47% <0.00%> (-11.31%)	⬇️
server/schedulers/base_scheduler.go	60.86% <0.00%> (-8.70%)	⬇️
server/schedulers/random_merge.go	61.19% <0.00%> (-2.99%)	⬇️
server/tso/tso.go	78.51% <0.00%> (-2.97%)	⬇️
server/server.go	75.41% <0.00%> (-1.21%)	⬇️
server/schedulers/adjacent_region.go	75.53% <0.00%> (-1.07%)	⬇️
client/client.go	71.60% <0.00%> (-0.51%)	⬇️
server/handler.go	50.53% <0.00%> (-0.44%)	⬇️
... and 5 more

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[lhy1024]

/merge

[ti-srebot]

Your auto merge job has been accepted, waiting for:

• 2636

[ti-srebot]

/run-all-tests

[HunDunDM]

LGTM

[lhy1024]

/merge

[ti-srebot]

Your auto merge job has been accepted, waiting for:

• 2640

[ti-srebot]

/run-all-tests